Assalaamu alaykum.
A bit of ambiguity in business is allowed, like all-you-can-eat buffets, but my phone contract says that I pay one fee and can use as many minutes as I want, is this ok given that I am sure that the network studied the usage and so on. Similarly, with Amazon Prime, I pay one delivery fee and get as many deliveries for free as I want, and Amazon loses on this; is this ok?

All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger.

Insignificant Jahaalah (ignorance and ambiguity) in contracts of financial transactions is excused and tolerated. This is a matter of fact in present-day transactions, and there is no harm in that; there is hardly a contract that is completely free of the element of Jahaalah. The scholarly difference in this regard is about the criterion in identifying the significant Jahaalah from the insignificant one. Al-Baaji wrote, "The insignificant Gharar (risk and uncertainty in transactions) does not impact the validity of a sale contract; no contract is absolutely free of such an element. However, scholars differed on the validity of contracts because of their difference of opinion regarding the extent of Gharar in a contract; is it significant and accordingly the contract is invalid, or is it insignificant and accordingly the contract is valid?" [Al-Muntaqa]

This applies to the element of Jahaalah in the amount of food consumed in an 'open buffet'; such Jahaalah is closer to being excused because people are tolerant about it. An-Nawawi said, "Muslim scholars unanimously agreed on the permissibility of things in which there is insignificant Gharar. For instance, they unanimously agreed that it is allowed to pay a fixed fee for entering public baths although people differ in their usage of water and the duration of their stay therein. They also agreed that it is allowed to drink from the water skin in return for a fee while the exact amount of the drunk water is unknown." Please refer to fatwa 135439.

As for the rest of the question, its answer is based on the fact that the knowledge that eliminates Jahaalah in Ijaarah (service) contracts is not attained by estimating the work or service only; rather, it is also attained by defining the duration of the service. Ibn Qudaamah said, "It is a condition to know the amount of the service, because the Ijaarah contract is a sale transaction in essence, and a sale is not valid unless the amount of the sold item is known. There are two ways to know it:

First: determining the required service, such as sewing a certain garment, riding a means of transportation, or carrying an item to a particular place.

Second: determining the duration of the service, such as leasing a house for one month..." [Al-Kaafi]

Al-Margheenaani said, "A service is sometimes determined by duration, such as renting a house for residence therein ... The contract is valid if its duration is specified ... It may be determined by naming the service itself, such as hiring a man to dye or sew one's garment ... It may also be determined by specification and indication, such as hiring a man to move such-and-such food to a particular place..." [Al-Bidaayah]

Hence, it is sufficient to determine the duration of subscription and the fees of that duration in order to establish knowledge of the service with regard to using phone lines, even if the number of used minutes differ. The same ruling applies to the shipping services if fixed fees are charged for a certain duration, contrary to when the duration is unspecified, in which case it would be Jahaalah that invalidates the contract and entails annulling it and conducting a new contract in which the amount or duration of the service is defined.

Allah knows best.